This invention relates to a magnetic recording and reproducing apparatus such as a time lapse magnetic recording and reproducing system (hereinafter referred to as a "time lapse VTR") for recording and reproducing picture signals while intermittently skipping frames, which is used for monitoring, burglar-proofing, and so forth.
Recently, a time lapse VTR which can make long time recording such as 720 hours on a single tape by sequentially switching picture signals from a plurality of video cameras and recording them while intermittently skipping frames has gained a wide application so as to simultaneously monitor various places such as plants, sites of construction, shops, etc., in place of guards. When any accident occurs, valuable information can be acquired by quickly reproducing the picture signals recorded on the tape.
JP-A-62-295581, for example, discloses a time lapse VTR which counts the number of tracks on a magnetic tape at the time of reproduction of the tape, extracts reproduction signals from the tape in one field or frame unit whenever the count value becomes equal to the number of video cameras, alternately records the outputs of a plurality of video cameras in one field or frame unit on the same magnetic tape, takes out the outputs of the desired video camera or cameras in one field or frame unit from the magnetic tape at the time of reproduction, and can thus confirm correctly the recorded content.
Next, the construction of a time lapse VTR using a video cassette tape of a VHS system and available commercially in the past will be explained by way of example with reference to the accompanying drawings.
FIG. 1 shows an example of the construction using the conventional time lapse VTR. In FIG. 1, reference numeral 1 denotes a time lapse VTR, 2 is a switcher and 3a to 3d are video cameras. Picture signals from the video cameras 3a to 3d (A, B, C and D ch (channels), respectively) are sequentially switched in a field or frame unit by a picture switching circuit 4 of the switcher 2 and are supplied to the time lapse VTR 1. Though the switcher 2 is shown separately disposed in an other apparatus, it may be assembled into the time lapse VTR 1. A chrominance signal in the inputted picture signals is converted to a low band signal by a color low-pass conversion circuit 7 in the time lapse VTR 1, while a luminance signal is subjected to frequency modulation by an FM modulation circuit 8. The outputs of these circuits 7 and 8 are added by an addition circuit 9, and each channel is sequentially recorded by four magnetic heads H.sub.1+ (12), H.sub.2- (13), H.sub.3- (14) and H.sub.4+ (15) mounted on a cylinder 11 on a magnetic tape 16, through a recording amplifier 10. In this case, operation modes include a normal speed mode (33.33 mm/sec in a standard mode of the VHS system, for example) for recording and reproduction while the tape is continuously transported at a normal speed, and an intermittent recording/reproduction mode (recording while the tape is stopped) for long time recording by intermittently transporting the tape. During reproduction, the reproduction head signals from the magnetic heads H.sub.1+ (12), H.sub.2 -(13), H.sub.3 -(14) and H.sub.4 +(15) are amplified by pre amplifiers 16, 17, 18 and 19, respectively, are then passed through switch circuits 20, 21, 22, are converted to chrominance signals and luminance signals restored to their original bands by a color high-pass conversion circuit 23 and an FM demodulation circuit 24, and are further added together by an addition circuit 25 to provide reproduction picture signals. System control means 27 activates a servo circuit 30 which controls the cylinder 11, a control head 28 and a capstan shaft 29 in accordance with an operation mode inputted by a user through mode setting/input means 26, and lets it control the operation condition of the time lapse VTR. The system control means 27 activates a change-over circuit 31, selects one of the outputs of the switch circuits 20 and 21 as the output of the switch circuit 22 in the normal speed reproduction mode of the tape, and selects the one of the outputs of the switch circuits 20 and 21 which has the higher level as the output of the switch circuit 22 under control of a level detection circuit 32 during a search reproduction mode in which the recorded content can be watched more quickly than the normal speed.
The switcher 2 divides the picture signal from the time lapse VTR 1 into field units by A/D means 33, an image memory 34, D/A means 35 and memory control means 36 which controls these circuit elements, and a reproduction picture of a desired channel can thus be obtained on a TV monitor 38 in accordance with the output of reproduction channel setting means 37.
Here, when a tape which is intermittently recorded is reproduced intermittently, a substantially flat and satisfactory reproduction head signal such as shown in (e) or (f) in FIG. 2 can be obtained on the basis of a SW30 signal 39 representing the period in which the magnetic heads 12 to 15 are in contact with the tape as shown in (b) in FIG. 2. Needless to say, a substantially flat and satisfactory reproduction head signal can likewise be obtained as shown in (e) or (f) in FIG. 2 when a tape recorded at the normal speed is reproduced at the normal speed. When the tape recorded intermittently is reproduced at the normal speed and thus is watched somewhat quickly, a reproduction head signal above a necessary level can be obtained, although the waveform becomes diamond-like as shown in FIG. 3 because the tilt of the recording track and the reproduction trace is different by one track. Accordingly, the reproduction picture of each of the A, B, C and D channels selected through the switcher 2 can be obtained satisfactorily in both of these cases.
In the conventional time lapse VTR described above, however, the reproduction trace orbit of the magnetic heads stretches over six tracks at the time of 5-time speed search as shown in FIG. 4 by way of example in a search reproduction mode in which a tape of intermittent travel multi-channel recording is run at a high speed. In this case, the outputs of the switch circuits 20 and 21 become diamond-like as shown in (f) and (g) in FIG. 5 and a level detection circuit 32 selects the output which has the higher level, so that the output of the switch circuit 22 becomes as shown in (h) in FIG. 5. Assuming that 4-channel picture signals are recorded by four cameras, the output of the switch circuit 22 includes A, B, C and D channels that partially exist in one reproduction field as shown in (i) in FIG. 5. In other words, each channel is reproduced partially on the TV monitor 38 as shown in FIG. 6 in such a manner as to correspond to the output of the switch circuit 22 shown in (h) in FIG. 5. Eventually, the A, B, C and D channels are multiplexed as shown in FIG. 7 and the resulting picture becomes extremely difficult to watch.